Project Summary Post-transcriptional gene regulation has emerged as a critical control mechanism that orchestrates diverse cellular processes during germ cell differentiation. We have extensive preliminary data that demonstrate cytoplasmic Rbfox1 represses the translation of specific target mRNAs that contain (U)GCAUG elements within their 3' UTRs during specific stages of germline cyst differentiation. Through this proposal, we seek to characterize the detailed mechanisms by which cytoplasmic Rbfox1 regulates stage specific mRNA translation programs during germ cell cyst development. Our general strategy is to (1) determine what mechanisms control the unique protein expression pattern of Rbfox1 during the intermediate stages of germline cyst differentiation, prior to the onset of meiosis, (2) characterize how Rbfox1 molecularly regulates the translation of mRNA targets to achieve stage-specific translational repression and (3) characterize how Rbfox1 regulates specific mRNAs that govern early steps of germ cell development, such as pumilio and sex-lethal, and determine the extent to which ectopic expression of these targets contributes to Rbfox1 mutant phenotypes. We anticipate that these efforts will provide key insights into the translational control hierarchies that promote the development of germ cells across species.